1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading an original image and forming image signals representing said original image.
2. Description of the Prior Art
There have been proposed various image processing apparatus for storage, transmission to a distant location or image recording on a printer, of image signals obtained by photoelectrically reading an original image with an image sensor such as a charge-coupled device (CCD).
In such image processing apparatus, unsharp mask data are often prepared from the original image for use, for example, in the edge enhancement of contours of characters in the original image, image discrimination for separating image areas and character areas mixedly present in the original image, or elimination of moire patterns encountered in reading halftone dot image or a fine-lined image.
In the printing field, for example, image signals and corresponding unsharp mask signals are simultaneously obtained by a mechanism shown in FIG. 1, in which an original 101 is wound around and is rotated with a drum 100. A scanned point A and the vicinity thereof of said original 101 are illuminated by the light from a light source 102, and the light reflected by the original is guided, through imaging lenses 103, 104, to photoelectric converters 105, 106. The optical systems including the lenses 103, 104 etc. are integrally moved along the rotary axis of the drum 100 to scan the original 100. A photoelectric converter 105 is placed at the focal point corresponding to the point A on the original 101, but the other photoelectric converter 106 is placed at an out-of-focus position relative to said point A or has a widened aperture in order to receive the image information of the point A and the vicinity thereof. In this manner the image signals and the corresponding unsharp signals are simultaneously obtained from the photoelectric converters 105, 106.
Though the image reading method shown in FIG. 1 allows one simultaneously to obtain the image signals and the corresponding unsharp signals on a point of an original under scanning, it is not applicable to a photoelectric converter with a linear array of photosensors, such as charge-coupled device (CCD).
The above-described optical formation of an unsharp image may be replaced by making an average of the image signals of an observed pixel and plural pixels therearound through a method (1) or (2) explained in the following:
(1) In the case of calculating image signals of n x n matrix obtained for example from a drum scanner, there will be required a buffer memory of a capacity of (l+n-1).times.(p+n-1) pixels, where l is the scanning width of the image and p is the number of photosensors capable of simultaneous image reading in the image reading apparatus, as shown in FIG. 2.
The value p is equal to 1 in the case of reading only one point with a photomultiplier, for example with a drum scanner as shown in FIG. 1. In such a conventional method, the capacity of the buffer memory has to be very large.
For example, in the case of reading an A4-sized original image with a photomultiplier with a resolving power of 10 lines/mm for a matrix calculation of 5.times.5 there will be required a buffer memory of (210mm.times.10 lines/mm+4).times.(1+4)=10,520 pixels.
(2) A better unsharp image signal for each sensor element of the CCD may be obtained by forming a two-dimensional image information by storing signals from the CCD in a memory, and applying a two dimensional filtering of n x m pixels, where n and m are integers to said two-dimensional image information.
This method, however, requires multiplying and summing operations of n x m times for obtaining an unsharp image signal. As an example, in the case of filtering with 13.times.13 pixels, there are required at least 169 multiplication-solutions and one division for obtaining the average value. Consequently, even if the data accessing and said multiplication-summation can be accomplished within a microsecond, there will be required at least 169 .mu. sec. for obtaining an unsharp image signal for an observed pixel. Also, the image reading time has to be extended since the scanned area is enlarged for obtaining the unsharp image signal, as will be explained later.
FIG. 3 shows an example of the image reading apparatus equipped with a photosensor array, in which an image reading head 107 is provided, as an example, with five photosensor elements. An original document 109 is moved in the vertical direction in FIG. 3 by a roller 111 driven by a motor 112, while the image reading head 107 is simultaneously moved in the horizontal direction by a motor 114 through a transmission member such as a belt or wire, and the image signals from the image reading head 107 are released after suitable processing in a data preparation unit 115.
FIG. 4(A) shows the scanning state in a single scanning motion of the image reading head 107 in the arrangement shown in FIG. 3 and the scanning area required for preparing the unsharp image signal with said image reading head 107. A photosensor array 116 of the image reading head 107 has five sensor elements a1-a5 linearly arranged in the subsidiary or secondary scanning direction and is moved from left to right in the main scanning direction to scan the original document 109. An image area 117 is read by the photosensor array 116 in one scanning motion, while an image area 118 is required for preparing unsharp image signals for the pixels contained in the image area 117.
FIG. 4(B) illustrates an example of the operation of forming an unsharp mask signal with said photosensor array 116. An unsharp mask signal b1 for a pixel al is for example obtained from the following equation: ##EQU1## wherein C.sub.i and C.sub.j are filtering coefficients for preparing unsharp mask signals.
Though a single scanning motion of the photosensor array 116 only provides data in an image area 117 indicated by A1, A2, A3 and A4, the calculation according to the equation (1) also requires image data P1-P10 positioned outside said image area. Thus, said calculation cannot be accomplished with the data of a single scanning of the photosensor array 116 but requires the data of the preceding scanning line as well. Similarly, for obtaining an unsharp mask signal for a pixel a5 at the right lower corner of the image area 117, there are required the image data of an area indicated by Q1, Q2, Q3 and Q4, including image data of the succeeding scanning line and those of a right-hand margin area.
The above-explained example shows a case of calculating the unsharp mask signal with 5.times.5 pixels, but in practice filtering with as many as 13.times.13 pixels may be used in order to prevent or detect moire patterns generated in the case that the original image 109 is a screentone dot image.
In the above-described method, there is required a memory of a capacity substantially corresponding to the image area 118 surrounded by R1, R2, R3 and R4 in FIG. 4(A) for unsharp image signal preparation, and the calculation of the unsharp image signals of the pixels in the image 117 of a scanning line indicated by Al, A2, A3 and A4 in FIGS. 4(A) and 4(B) is made possible only after the sensor array 116 has read three scanning lines.
As explained in the foregoing, the conventional method for obtaining unsharp mask signals with a linear sensor such as CCD has been associated with the drawbacks of:
(1) requiring an extremely large memory capacity; PA1 (2) requiring a very long calculating time for obtaining the unsharp mask signals; PA1 (3) requiring a long image reading time, since the unsharp mask signals can be obtained only after the scanning is made wider than the effective image area in the vertical and horizontal directions; and PA1 (4) requiring complex control means since the image area required for preparing unsharp mask signals is different from the image area required for obtaining image signals.
Also, the discrimination of an image has been made by storing two-dimensional image information, obtained with an image sensor from an original image, in a memory and calculating the average value or frequency distribution of a particular area on said memory.
Such process requires a memory of a large capacity for storing the image information form the original image, and a high-speed processing is required in case the input and output are connected on real-time basis. For these reasons such image reading apparatus has been associated with a difficulty in the circuit structure as well as in the magnitude of the circuitry.